One thing I’ve been thinking about this week is my grandpa’s sweat.

Well, not specifically my grandpa’s sweat, but the words of an intro to a YouTube video I watched many years ago. Despite scouring the internet for this video, I can’t find it, but it has nonetheless stuck with me: a YouTuber started a video in front of a lake and said that his grandpa’s sweat was in it.

His grandfather had swum in the lake only once, decades earlier, but claimed that after all those years, trace particles of his grandpa’s sweat had been mixed around and still remained. If he were to take a sample of water, there would be decades-old minerals, in minute quantities, that were there only because his grandpa swam in the lake many years ago.

The science behind this was a bit outside my specialty, so I asked Paul Meyer Reimer, physics professor, what he thought — is that really happening?

“Yes,” Meyer Reimer said, and gave an example: “For Lake Michigan, it should take about 23 centuries” for any amount of water to leave the lake.

Water leaves a lake either due to evaporation or outflow, and minerals can leave only through outflow. In a simplified model, the magnesium, sodium, lactic acid and any other minerals or unevaporated water would be in the lake for 2,320 years until “all the water turns over in the lake,” according to Meyer Reimer.

There are a few assumptions made in the calculation: some salt and minerals might sink to the bottom and be absorbed by the underwater flora, and evaporation complicates the calculation, but as Meyer Reimer said, “the physicist’s approach is to simplify as much as possible.”

I’m not sure which lake the YouTube video (or perhaps clip from a middle-school science lesson) was referring to — each lake has a different outflow or turnover rate. The point remains though, that, as the YouTuber said, sweat will stay in a lake for many years.

So, if one person’s swim in a lake has forever changed its chemical makeup, how many more indelible marks have we left on the world? When our bodies interface with our physical surroundings, we leave marks on it, just as we too come away changed.

Meyer Reimer pointed out another example of this: how many molecules of Caesar’s last breath do we inhale every time we breathe? According to Lawrence Weinstein and John Adam’s book “Guesstimation,” it’s about eight molecules — and eight more of George Washington’s last breath, eight from Cleopatra, and so on.

Here’s some more thought experiments on the longevity of our actions: Trees get their mass from carbon dioxide, so whose exhalations make up the tree I swing from? And water cannot be destroyed, so have I drunk any molecules of water also swallowed by dinosaurs?

I think one reason that the lake-sweat video stuck with me for so long has to do with the idea of our lasting impacts. In ways large and small, often unnoticed yet still observable, we are altering our surroundings.

The particulars of our actions will fade, sodium will fall into solution with lake water and water will continue its cycle through the clouds and our pores, as the rhythms of the Earth nudge up against us and we gently nudge back.